What is the role of the environment variable classpath?

Classpath is an environment variable dedicated to the javac compiler. Its function is to tell the Java execution environment in which directories the classes or packages required for the Java program you want to execute can be found.

Environment variables (environment variables) generally refer to some parameters used in the operating system to specify the operating environment of the operating system, such as: Temporary folder location and system folder location, etc.

An environment variable is an object with a specific name in the operating system, which contains information that will be used by one or more applications. For example, the path environment variable in Windows and DOS operating systems, when the system is asked to run a program without telling it the full path where the program is located, the system should not only search for the program in the current directory, but also go to the path specified in path. . Users can better run processes by setting environment variables.

Classpath is an environment variable of the javac compiler. Its function is related to the import and package keywords. The purpose of setting Classpath is to specify the class search path. To use the classes that have been written, the prerequisite is of course that they can be found. The JVM uses CLASSPTH to find the .class file of the class. We need to set dt.jar and tools.jar in the lib subdirectory under the jdk installation directory to CLASSPATH. Of course, the current directory "." must also be added to this variable.

When you write improt java.util.*, when the compiler faces the import keyword, it knows that you want to introduce the class in the java.util package; but the compiler How do you know where you put this package? So you first have to tell the compiler where the package is located; how to tell it? Just set CLASSPATH!

If the classes in the java.util package are in the c:\jdk\ directory, you have to set the c:\jdk\ path to CLASSPATH! In this way, when the compiler faces the statement import java.util.*, it will first search the directory specified by CLASSPATH, check whether the subdirectory java\util exists, and then find the compiled file (.class file) with the matching name ). If it is not found, an error will be reported!

When you develop a package yourself, and then want to use the classes in this package; naturally, you have to set the directory where the package is located to CLASSPATH!

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